The purpose of ancova to compare two or more regression lines. It is a way
of comparing the Y variable among groups while statistically controlling for
variation in Y caused by variation in the X variable. For example, let's say
you want to know whether the Cope's gray treefrog, Hyla chrysoscelis, has a
different calling rate than the eastern gray treefrog, Hyla versicolor,
which has twice as many chromosomes as H. chrysoscelis but is
morphologically identical. As shown on the regression web page, the calling
rate of eastern gray treefrogs is correlated with temperature, so you need
to control for that.